The discovery of the fluidized bed process for the production of polymers provided a means for producing these diverse and widely used polymers with a drastic reduction in capital investment and a dramatic reduction in energy requirements as compared to then conventional processes. The present invention provides a means for even greater savings in energy and capital cost by affording a simple and efficient means for obtaining a substantial increase in production rate in a given size reactor over what was previously possible in a fluidized bed process.
The most common and perhaps universal means of heat removal employed in conventional fluidized bed reactor processes is by compression and cooling of the recycle gas stream at a point external to the reactor. In commercial scale fluidized bed reaction systems for producing polymers such as polyethylene, the amount of fluid which must be circulated to remove the heat of polymerization is greater than the amount of fluid required for support of the fluidized bed and for adequate solids mixing in the fluidized bed. The fluid velocity in the reactor is limited to prevent excessive entrainment of solids. A constant bed temperature will result if the heat generated by the polymerization reaction (which is proportional to the polymer production rate) is equal to the heat absorbed by the fluidizing stream as it passes through the bed, plus any heat removed or lost by other means.
Unfortunately, it has long been believed that the recycle gas temperature could not be lowered any further than to a point slightly above the dew point of the recycle gas stream. The dew point is that temperature at which liquid condensate begins to form in the gas stream. Common practice has been to limit the temperature of the recycle stream at the outlet of the cycle heat exchange zone to a temperature at least about 3.degree. to 10.degree. C. above its dew point (see copending U.S. patent application Ser. No. 49,555 of June 18, 1979, now abandoned, equivalent to published European Patent Specification No. 0 021 605, page 22, lines 8-22). This assumption was predicated on the belief that the introduction of liquid into a gas phase fluidized bed reactor would inevitably result in plugging of the distribution plate, if one is employed; non-uniformity of monomer concentrations inside the fluidized bed and accumulation of liquid at the bottom of the reactor which would interfere with continuous operation or result in complete reactor shut-down. For products, such as those using hexene as a comonomer, the relatively high dew point of the recycle stream has until now severely restricted the production rate.
The primary limitation on reaction rate in a fluidized bed reactor is the rate at which heat can be removed from the polymerization zone. Although they differ in very important ways from gas fluidized bed reaction systems, the same heat limitation problems exist in other types of reaction systems such as stirred reaction systems and to some extent, slurry reaction systems.
In U.S. Pat. No. 3,256,263, heat removal in a stirred reaction system is achieved by the compression of recycle gases and expansion upon reentry into the reactor. In other stirred or paddle-type reaction systems some additional cooling is effected by the injection of liquid onto the top of the bed. See for example U.S. Pat. Nos. 3,254,070; 3,300,457 and 3,652,527.
In U.S. Pat. Nos. 3,965,083; 3,970,611 and 3,971,768 assigned to Standard Oil Co., cooling of a stirred bed reactor is supplemented by injection of liquids on the top of the bed.
In U.S Pat. No. 4,012,573 (Trieschmann et al.) gases withdrawn from a stirred reactor are condensed to liquid and returned in liquid form to the stirred reactor where the liquid is brought into desired contact with polymer in the stirred bed.
Mitsubishi Petrochemical Co. has proposed the use of liquids or regasified liquids for cooling in a gas phase reactor (J No. 55/045,744/80 and DT No. 2 139 182). In both of these descriptions the liquid or regasified liquid is injected into the bed rather than entering with the fluidizing gas as in the present invention. DT No. 2 139 182 is specific to stirred beds rather than fluidized beds. In J No. 55/045,744/80 the liquid is regasified before being injected into the fluidized bed.
In a fluidized bed reaction system, as distinguished from stirred or paddle-type reaction systems, uniform distribution of monomer and catalysts in the upwardly moving gas stream is essential to avoid hot spots and resulting polymer chunks. In stirred and paddle-type reactors these problems are overcome by mechanical stirring and agitation. A further requirement of a fluidized bed reactor system is that the velocity of gas flowing through the reactor be adequate to maintain the bed in a fluidized state. The gas velocity required to keep the bed in a fluidized suspension cannot be achieved under normal conditions by mere injection of liquid at the bottom of the bed. Therefore, the direct liquid injection cooling of a reactor, as described by Treschman et al. is not a viable option for a fluidized bed reaction system.